The present invention relates to a transmission unit for a motor vehicle, having an electrical machine that acts as both a starter for the engine and a generator. More precisely, the invention relates to a transmission unit for a motor vehicle with a drive train that is intended for coupling to an internal combustion engine on the one hand and to the wheels of the vehicle on the other, and having an electrical machine and a transmission, which is constructed for transmitting torque from the electrical machine to the drive train and vice versa.
Such transmission units are known, but despite their advantages such as weight, space and cost reduction that are associated with the omission of an electrical machine, they have still not become widely established.
In modern motor vehicle, separate starters and generators are typically always installed. The starters are shifted via a thrust wheel only as needed, and as a result their moment of inertia as no effect on the dynamics of the drive train. Modern generators are typically connected to the engine crankshaft via pulleys, and their rpm therefore closely follows that of the crankshaft. As a result, each time the engine is braked, torque is transmitted back from the generator to the drive train, and in other words the generator is unnecessarily braked as well. In this way, energy is uselessly lost upon each braking, and furthermore the rigidly coupled generator that also rotates increases the braking moment that has to brought to bear, which is also undesirable.
To solve this problem, internal combustion engine and generator units have been developed in which so-called free- wheel pulleys are used. These free-wheel pulleys decouple the generator if the generator shaft rpm is (even only briefly) above the crankshaft rpm of the engine at the same instant.
These free-wheel pulleys therefore permit a torque transmission only from the engine to the generator, but not vice versa. They are accordingly unsuitable for an electrical machine that is intended to act as both a starter and a generator.
A transmission unit for a motor vehicle is created that makes it possible to unite the functions of the starter and the generator in a single electrical machine, and nevertheless it offers the advantages in terms of effective braking and the avoidance of energy losses that can be attained, in conventional generators coupled to an internal combustion engine, by the known free-wheel pulleys.
The transmission of a unit of this kind expediently has two operational switching states, one for the torque transmission via the drive train to the electrical machine and one for the transmission in the opposite direction, which states differ in their gear ratios.
Expediently, a control circuit is provided, which as a function of a detected deceleration of the motor vehicle puts the transmission into the idling switching state without further human action. To that end, the control circuit advantageously monitors the rpm of the drive train. This can be done directly or indirectly, for instance by monitoring the rotary speeds of the wheels of the motor vehicle. To that end, the control circuit can cooperate with an ABS control unit.
Whenever the exceeding of a predetermined acceleration of the motor vehicle is detected in this way, the control circuit puts the transmission in the idling switching state. This decouples the electrical machine from the drive train so that it need not follow the braking of the drive train. Instead, kinetic energy stored in the rotating parts of the electrical machine can continue to be used for generating current.
The control circuit furthermore expediently also monitors the rpm of the electrical machine, and it switches the transmission back from the idling switching state to an operational switching state when this rpm drops below a limit value. For determining this rpm, a signal of a frequency converter can be used; such a converter is typically connected to the electrical machine of a transmission unit of the type defined at the outset, in order to rectify the current generator by the electrical machine and enable its storage by a vehicle battery.
The limit value below which the control circuit shifts the transmission back to the operational switching state is advantageously defined dynamically and in proportion to the ascertained rpm of the shaft, on the side toward the drive train, of the transmission. The idling switching state is maintained until such time as the electrical machine rotates faster than what corresponds, taking into account the gear ratio of the operating, or operational, state, to the rpm of the shaft, toward the drive train, of the transmission. Not until the rpm of the electrical machine has dropped below this value, either because it has been braked by generation of electrical power or because the vehicle is being accelerated again, is a switch back to the operating state made. This precludes an undesired torque transmission from the electrical machine to the drive train.
Further characteristics and advantages of the invention will become apparent from the ensuing description of an exemplary embodiment.